Somewhat OT: debugging circuit

[nosamiam]

Hey y'all,

I haven't been around here much, being more of a keyboard player these days.  But you all have helped me out in the past on my pedal builds and there is definitely some crossover in what I'm about to ask.  The same issue could be had in a pedal, I'm pretty sure.  I've debugged several of my own and a few belonging to other people, even my guitarists Fuzz Face while the crowd was waiting for us to go on! But that's a different story for a different thread.

I have an ARP analog synthesizer that has a problem.  One of the two oscillators is not working.  I'm trying to use a DMM and an audio probe to debug it.  My question is this:

Using the tone generator function of the DMM:
-I have the ground of my DMM and the ground of my audio probe clipped to PCB ground.
-I touch the DMM signal lead to my audio probe signal lead together and I get tone from my speaker.
-If I touch them to adjacent points (or even the SAME point) in the audio path on the PCB I get nothing.

Does this mean my audio path is shorted to ground?  If so, how do I isolate the bum component.  This is driving me NUTS!

I appreciate any help on this.  Or even just words of condolence.

[nosamiam]

Bump.

Nothing?

Ok, ignore the talk about oscillators and such.  Just pretend this is a pedal that has no output and you're using an audio probe and a tone generator to debug. Any clue?

[petemoore]

  Start at the input with a signal, tone generator is good.
  Use the audio probe there, at that point: hear sound it works..the debugging tools anyway.
  Follow the signal path wire farther into the circuit, probe again...hear sound..the wires and whatever is betweent he injector and the detector [generator and audio probe] probably works.
  I say probably because if it's a gain stage set for gain, you should hear a gain increase [more volume at the output of the stage].
  If at some point you hear a volume drop or complete signal loss, something like a volume control working correctly or a miswire is causing it...so unless there is supposed to be a volume drop..there's a problem.
  At that point the probe has done it's job [once..so far] identifying a problem stage or whatever, and taking voltage measurements of the problem area helps narrow down the actual fix.

[nosamiam]

Thank you so much, Mr. Moore.  I appreciate you replying and helping me out.

I've used the audio probe successfully in the past and I'm pretty familiar with how it works.  This situation is weird though...

I can't find a point in the signal path that will conduct signal at all, from beginning to end (and it's a loooooong path).  Does this mean that the entire circuit is possibly shorted to ground?  If I touch the tone generator probe directly to the audio probe (leaving the circuit out completely), I get signal.  If I touch both to any 2 points in the signal path, or even a single point, I get nothing.

[petemoore]

  First what's the voltage of the supply?
  And can you measure and drain it if necessary.
  ok..don't put hand in it...
  Connect a cable from output to the amp, turn the amp up enough so you'll be able to hear...
  Then tap an insulated metal screwdriver to the tip of the jack with the cable to amp in it, you should hear a click, or put a source on it, tone generator or what have you, although the output may load the source signal, there should be enough to make the amp sound.
  If you hear signal coming through the amp the jack is at least debugged.
  If not test for ground on the tip lug, and measure DC resistance from tip to ground.
  Once you can use the jack to get signal through the chassis, move the next point 'to the left' [toward input near output..] there's probably a volume control and DC blocking cap in that area, see if you can get signal into the amp through them.
  Next left item is likely an active component or tone control...work your way toward the input, see if you can find a schematic or ferret out where [if you find where] signal is being lost.
  I'm not sure what the voltages are in the unit, so I really should just suggest a technician for it.
  See debugging thread at top of threads, most/ all the info there applies.

[nosamiam]

Thanks again Petemoore.  I can rule out the jack as the problem because this synth has 2 oscillators (read: built-in tone generators) and one of them works fine.  Like I was saying earlier, at no point in oscillator 1's signal path can I get it to carry audio.  There are parts where the two signal paths (osc1 and osc2) are shared which makes it even trickier.

The circuit uses a lot of 4558's and I checked voltages at all of those: +/- 12V at all of them.  I'm gonna have to dig up datasheets on the rest of them.

Oh well, thanks for your help.  I'll keep plugging away and I'm sure I'll figure it out.  If not, like you said, I'll send it off for fixin'. Thanks!!